Selective squelch circuit



March 25 1952 G. D. HANCHETT, JR

SELECTIVE sQ'UELcx-xv CIRCUIT Filed Aug; r2o, 1947 Patented Mar. 25, 1952 UNITED STATES PATENT OFFICE SELECTIVE SQUELCH CIRCUIT George D. Hanchett, Jr., Millburn, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application August 2o, 1947, serial No. 769,600

Claims.

My present invention relates generally to electronic selective squelch circuits, and more particularly to an improved squelch circuit adapted to be selectively responsive to a predetermined tone signal.

In mobile communication Work, such as used for 'example by municipal police and fire departments,'it is desirable to have some method by which a particular receiver, or a group of receivers, in the mobile units can be selected for radio communication. In the past such selective.` communication has been provided by relatively complicated circuits both at the transmitter and at the desired receivers.

It is an important object of my present invention to provide a selective radio communication system which utilizes at the desired receiving point an electronic selective squelch circuit employinga thyratron squelch tube.

In accordance with another object of my` invention there is provided an audio frequency amplifier Whose output tube has its plate current consumption controlled by an electronic switch device thereby to provide a substantial economy in the total receiver B supply current.

It is another object of my invention to provide in combination with a source of audio frequency energy, which energy includes a tone signal of a frequency above the highest usable audio signal frequency, an audio frequency amplilier network adapted to be controlled in its operation by an electronic switch device having its own operation dependent upon ay selective controlled system which is responsive to the aforesaid tone, wherebythe audio amplifier will not require any B supply power during such periods of time when audio frequency energy is not supplied to it.

From a more specific viewpoint, in accordance with my present invention, if a dispatcher at a fixed or main station desires to contact a particular mobile unit, he will select a frequency for tone modulation that will be above the normal voice communication frequencies, and to which the particular desired mobile receiver is tuned.

Thistone after detection in thev receiver is passed to a resistance tuned amplifier, and then on tc -duce in gain value.

One added advantage of the present system is l Avthe' f-act that the squelch is effective on the output amplifier.y This is a power saving scheme,

since the output amplifier requires a large proportion of the total receiver B supply current. 4 Stand-by periods then will not require any B supply power to the output amplifier. The thyratron squelch tube employs a selective input circuit which is made up of a pair of triodes whose space current paths are arranged in series, there being included a degenerative feedback path between the plate circuit of one triode and the grid circuit of its associated triode. The feed back path includes a resistance-capacity bridge circuit having a characteristic which shows a sharp rejection dip at a predetermined frequency.

Still other features and objects of my invention will best be understood by reference to the following description taken in connection with the drawing, in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into effect.

Referring now to the accompanying drawing, which shows a circuit embodying my present invention, the numeral I designates the conductors leading to any desired source of audio frequency signals. As stated heretofore, the source of audio frequency signals may be the detector of a mobile receiver with which a fixed transmitter is seeking radio communication. On the other hand the source of audio frequency signals may be a line transmitting the audio energy. or it may even be a public address system, such as used in a hospital, school, hotel, railroad station and the like, where it is desired to provide selective announcements over various loudspeakers at cer-v tain times, where as at other times itis desired to have the loudspeakers silent.

Assuming for the purposes of this application thatv the tube 2 is the audio output amplifier tube of a mobile receiver, it is pointed out that in such case the transmitter, whether it be mobile or xed, will modulate its carrier with the normal voice frequencies, but will include on the carrier a specific tone signal. For example, if there are a dozen police radio patrol cars in a given area which are in communication with a transmitter, each receiver will have a specifically different tone frequency assigned to it. Since the band from 30 cycles to 5000 cycles covers the normal voice communication, then each of the mobile receivers will employ a respective tone frequency of from 6000 cycles to 18,000 cycles. If at the main station it is desired to contact a particular mobile unit, say 6000 cycles, at the transmitter there will p be applied to the carrier to which all the mobile receivers are tuned the normal voice cornmlmication frequencies in the band up to 5000 cycles, but, in addition, there will be a 6000 tone modulation applied to the carrier. On the other hand, if it is desired to transmit a general call to all mobile receivers, then all the tone signals from 6000 to 18,000 cycles will be applied to the carrier with the voice modulation frequencies. At each receiver I have provided means for having the audio amplifier system rendered operative solely in response to the emission from the main station of its carrier modulated with the tone frequency of the particular mobile receiver. In this way by proper selection and tuning of the receivers it is possible to secure single station to station communication.

It will obviously be understood that unless the audio frequency signals applied to the conductors I include a predetermined tone modulation frequency, regardless of the source of the audio signals, the output amplier tube 2 will not become operative. In the case of the amplifier being ineluded in a mobile receiver as explained above, the received modulated carrier will, of course, be selective by proper tuning and amplification, and after detection to secure all the modulation signals on the carrier, the modulation voltage is applied to conductors I. In the case of a public address system the output of the microphone of the system will include a super-audible tone frequency, say above 6000 cycles, for controllingthe y operation of amplifier 2 while the audio signals are applied to conductors I.

The circuits involved in the system are shown in the drawing between the conductors I and the amplier tube 2. Numeral 3 designates the audio frequency output transformer coupling the plate 4 of tube 2 to the voice coll of a suitable loudspeaker (not shown) The primary Winding 5 of transformer 3 is included in circuit with plate 4 and the B+ termin'al of the power supply network (not shown) The cathode 6 is of the indirectly heated type, and it is to be understood that its heating filament 1 will be connected, as indicated by letters x, to the A supply lines 8. The A supply line may be the usual 6 volts rated supply circuit, and in the case of a mobile receiver unit will be a suitable portion of the generator system of the car. The B+ terminal, of course, Will be located at a suitable point on the rectified output of the generator supply circuit. Those I skilled in the art of radio communication are well acquainted with the circuits used to energize radio receiving systems, and it is not believed necessistor 8', winding 9, lead I0, plate II of thyratron tube I2, cathode I3 of the thyratron tube and lead 1 I4. The grounded line of the system is indicated by the numeral I5. It will, therefore, be seen that the space current path of thyratron tube I2 completes the space current circuit of amplifier tube 2. It is obvious that when there is no conduction between the cathode and plate of thyratron tube I2, then the output amplifier tube 2 is cut off, and vice versa. The winding 9 is another of the windf ings on one of the power transformers of the power supply system. Hence, there exists across winding 9 alternating current voltage, say about 10 volts peak. For example, the alternating current voltage across winding 9 may be the domestic 60 cycles, which means that the plate I I of thyratron tube I2 will have applied thereto the alternating current voltage existing across winding 9. In this way the tube I2 is under the control of the periodic voltage variations at winding 9. The frequency of the voltage across winding 9 will be cycles where the voltage is derived from the natural period of the vibrator when used in mobile work.

The signal input grid I6 of tube 2 is connected to ground through the resistor I1. The cathode end of resistor 8 is connected to ground through condenser I8 whose upper electrode is indicated as having a positive potential. It is, also, indicated that substantially 16 volts are developed across condenser I8 as a negative bias for the grid I6. Condenser I8 is the cathode bypass condenser. The grid IIS is coupled to the source of audio frequency signals I through a path which includes a low pass filter I9 and audio frequency coupling condenser 20. Briefly, the low pass filter passes all audio frequencies between zero to 5000 cycles, which means that all tone frequencies above 5000 cycles will be rejected. Since the voice communication frequencies are all in the audio range between 30 cycles and frequencies considerably less than 5000 cycles, it follows that there will not be applied to signal grid I6 any of the control tone components.

Considering now, the thyratron control tube I2 and its selective input circuit, the thyratron tube is, of course, a gas tube as indicated schematically by the black dot adjacent cathode I3. The heating filament 2I has one of its leads 22 connected to one side of the A supply, While the second lead of filament 2I is connected through conductors I4 and I4' to the opposite side of the A supply circuit. Since the cathode I3 is connected to one side of the A supply circuit, the control grid 23 of tube I2 may be connected to the lead 22 to establish a predetermined negative bias for thyratron grid 23. This is done by connecting grid 23 to the conductor 22 through the path comprising resistor 24 and resistor 25 in series. It will be understood, therefore, that grid 23 has a normal -6 volts bias with respect to cathode I3.

The selective input circuit of the thyratron tube I2 generally consists of a degenerative system which functions to invert the impedance characteristic provided by the resistance-capacitor bridge Ti. The bridge T1 is a twin T resistance bridge which normally possesses a sharp frequency rejection characteristic at the predetermined tone frequency to which it is desired that the audio amplifier be operative. For example, if the tone frequency in the given example were 6000 cycles, then the bridge T1 normally has sharp rejection dip at 6000 cycles. This type of resistance bridge circuit is known by itself.

One of the T sections of the bridge consists of resistor arms 26 and 21 and the condenser 28, having its ungrounded electrode connected to the junction of resistor arms 26 and 21. The second T section of the bridge consists of a condenser arm 29, a condenser arm 30 and a resistor 3l whose ungrounded end is connected to the junction of condenser arms 29 and 30. I prefer to divide resistor arm 21 in two sections, and section 21 is made adjustable so as to vary the frequency response of the bridge T1.

The junction of arms 21 and 29 is connected through a coupling condenser 32 to the input electrode of the direct current blocking condenser 33 whose output electrode is connected to the junction of resistors 24 and 25. The junction of arms .fagtcogaio z'e hiam-'d vau 'connected through resistor R34 "to "thegrcundline 1'5.

' r'Thebridge Tiis'itself connected in'a'de'generaltive "feed-back Apath between apair "of -triodes vvhose'sp'ace current 'paths are 'arranged'in aseries connection. Thus, I have Ashown 'the 'pairof ftio'desasincluded in a commontwin triode tube 35, although separatetubes maybe utilized. 'In addition, I am notrestrictedto the use 'of triodes, since any other suitabletype of tube maybe employed. The input triode has its input grid 36 connected'through the'audio coupling condenser `J{I1`to the ungrounded conductor I of the audio frequency signal source. In other words., `there #is .appliedto the control grid 36 the entireaudio voltage, both voice modulationirequencies and control tone, existing .at "the audio source. The plate 3B of vthe input triode is connected tothe B+ Vterminal through a suitable resistor 39, Vwhile rthe cathode `4I) is connected to lead 4I through a suitable bypassed bias resistor 42.

The control grid 35 is connected through re- '.sistor 43 to the low potential end of bias resistor 42 'whereby the input triode functions as an audio amplier. The series triode of tube 35 has its cathode 4d connected to the grounded line I5 ythrougha suitably bypassed bias resistor 45, while ithe plate 4t is connected to the low potential end of bias resistor d2. In other Words, the space current of the input triode of tube 35 flows through the second triode between plate 46 and Vcathode v4t to ground. The control grid 41 is connected to thehigh potential end or resistor 34 through lead 48. It will, therefore, be seen that anormal negative bias is applied to grid 41 which `is equal kto the voltage developed `across bias. re- `sistor t5. In addition, Ithere is applied to grid 41 in degenerative phase voltage developed across the bridge T1.

The voltage developed across bridge T1 is, of course, an increasing quantity on either side of Athe illustrative 6000 cycle frequency by virtue of the rejection characteristic of the bridge. The degenerative `circuit is 100% degenerative vfor all frequencies except the null (6000 cycle) frequency of the bridge. Accordingly, there will be applied from the plate circuit of the `input triode only voltage which has the null frequency, which is :the desired 6000 cycle control tone frequency. It is .pointed out that by means of the degenerative TVfeedback across the bridge T1 to the series triode 4'4, 41, 46 there has been provided an Vinversion of the impedance characteristic of the bridge. That is to say, the resultant effect of the feedback arrangement of the series triode is to lprovide the thyratron 'input electrode with aselective "input circuit which has a very sharp peak at the control' tone frequency.

Considering, now, the operation of the thyratron circuit, and more particularly its function as a-n electronic switch in the cathode circuit of the output amplifier tube 2, it is rst pointedl out vtl'ia't'tube I2 'is normally non-conductive by virtue of the norm-al -6 volts bias 'applied to kgrid 23. Itis'not .believed necessary to describe in detail the functioning of a gas tube of the thyratron type. The grid 23', which is tied to the cathode I3, sutilized Vto secure the normal sharp on-off characteristic of the thyratron. Actually, the

tube functions in the manner of a grid-controlled rectiiier of alternating current voltage. Infthis case thealternating voltage at 60 cycles is'applied to theplate II.

Upon the application of .the controltone voltage through condenser 33 to grid 23 the gas tube tone lvoltage =is applied to grid 23 the thyiatron tube 'remains'inconduc'tive condition despite the application of the alternating voltage to the plate II. However, should `the control tone voltage be removed from `grid '23, then the tube I2 will immediately become non-conductive. This is due to the fact that the plate I I is 'rendered inactive on thenegative halif cycles of the alternating current voltage existing at winding 9. If `a tone signal is applied to grid`23 the thyratron vtube I2 will conduct on each positive half cycle of the voltage from winding Q, and will extinguish on each negative half cycle. The current, therefore, appearing through condenser I8 will be periodic, but the value of capacity o'lf condenser IS is suiiicient to smooth the voltage to a point where the Vgrid bias of tube 2 iis substantially steady.

It will, therefore, be seen that the function of coil or winding 9 is periodicallyto sense, or seek, to determine whether a, tone voltage is applied to grid 23 of the thyratron tube. If the tone voltage does exist at grid 23, then during the positive half cycles of the voltage or plate II the tube remains conductive. The removal of the tone voltage results in the thyratron being rendered non-conductive, as explained before. It will, now, be seen that the resistance 8, Winding vS. capacitance I3 and the thyratron tube I2 provide the cathode biasing circuit of the output ampliiier tube.

`When the thyratron is conducting it will have a denite potential drop from its anode II to cathode I3 of approximately 8 volts in most cases. The resistance 8 is used in series with this potential drop to produce the proper operating bias for the output tube. The function orf the winding 9 is periodically Ito throw the plate II below the sustaining ionization potential. Hence, if the signal from the main transmitter has ceased, then the next cycle that the anode II of the thyra-tron is driven negative will de-ionize the thyratron tube and its grid will again `gain control of the tube.

One of the main advantages of this system is that the thyratron squelch tube willnot be actuated, or rendered conductive, unless there is applied to the conductors I a desired tone frequency. In other words if the bridge Tris adjusted for a desired tone frequency, say 6000 cycles, then a tone of 6500 or '1000 cycles will not open the squelch tube I2. Of course, if the transmitter carrier has all tone frequencies thereon, then all squelch tubes a-t the various receiverswill become conductive.

Another important advantage of this system is the fact that the squelch tube operates at the cathode of the output amplifier tube. This is a power saving arrangement, since the output amplifier Vtube requires a large proportion of the total receiver B supply current. `Stand-by periods will not require any B supply power to the output amplier tube. In this way it is possible to eliminate the usual driver -tube and class B stage usually employed inamplitude modulated receivers of the mobile type. All that is necessary in the operation of the present system is that the relatively small A supply current be expended to heat the iilaments of tubes I2 and 2.

It is pointed out that by cutting out the plate current consumption of the output tube 2 during stand-by periods, as muchas Vs to '1/2 of the B supply power is economized. Inthis'waythere 'is secured economy of power consumption, the

2 may be operated as a class A audio ampliiier.

This system may be used with either frequency or amplitude modulated receivers.

While I have indicated and described a system for carrying my invention into eiect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modications may be made without departing from the scope of my invention.

What I claim is:

1. Invcombination with a source of audio frequency signals to be amplified in which a control tone signal of a predetermined high audio frequency may be selectively introduced, an amplier tulbe for said signals to be amplified having input and output electrodes, filter means between said source and said input electrodes effective to apply only the signals to be amplified to said input electrodes, said output electrodes being adapted for connection to an audio utilization circuit, a direct current voltage supply circuit including the space current pathof said tube, van electronic control device included serially in circuit with said space current path of said tube, and a. second filter means responsive to said control tone frequency connected between said source and said electronic control device and effective to apply ltube and consequent ampliction thereby of the signals to be amplified.

2. In a system as defined in claim 1, said electronic device being a thyratron gas tube, said gas tube having its Space current path arranged in series withzthe space current path of said audio amplier tube.

3. In a system as dened in claim 2, a source of alternating current voltage in circuit with the space current paths of said thyratron gas tube and said audio amplifier tube.

4. In an audio amplifier for audio frequency signals, a direct current voltage energizing circuit connected to said amplifier, va thyratron gas tube connected in series in the cathode circuit of said amplifier and functioning as a stand-by switch to economize energizing current, a selective resistance-capacity input circuit for said thyratron tube, tuned to a frequency which is equal to thelfrequency of a control tone signal included in said audio signals eecting operation of said thyratron tube and said audio amplifier only upon inclusion of said control tone signal in said audio frequency signals.

5. In an electronic tube amplifier, the combination with an amplifier stage thereof having a cathode return circuit, of a grid-controlled gas tube having input electrodes and output electrodes connected with said cathode return circuit their space current paths arranged in series, means for applying a control signal voltage to the input electrode of one of the series connected tubes, ani impedance network having a predetermined frequency rejection characteristic arranged of said one tube, said impedance network being included between the input electrodes of said gas tube, a degenerative [feedback circuit connected from said impedance network to the control grid of the second of said series connected tubes, a source of signalsto be amplified in which a control signal may be selectively included, filter means effective to permit only the signals to be amplified to be applied to said amplier stage, and said impedance network being effective to permit only said control signal voltage to be applied to the input electrodes of lsaid gas tube, whereby said gas tube is rendered conductive and said amplifier stage is rendered operable to amplify the signals tobe amplified upon the application of said control signal voltage to the input electrodes of said gas tube.

6. In combination with a source of signals to ,be amplified which include a control signal of a predetermined higher frequency, an amplifier tube having input and output electrodes, said source being coupled to the input electrodes, means coupling said source and said input electrodes effecting the application of the signals to be amplified to said input electrodes, said output electrodes being adapted for connection to a signal utilization circuit, a Voltage supply circuit including the space current path of said tube, an electronic control device included serially in circuit with said space current path of said tube, ya frequency selective network responsive to said control signal frequency for controlling the eiect of said electronic control device, coupling means eiiecting the application of control signal currents with the signa1 currents to be amplied to said frequency selective network, and said frequency selective network being effective to permit only the control signal voltage to be applied to said electronic discharge device.

7. In a system as defined in claim 6, 'said electronic device being a thyratron gas tube, said gas tube having its space current path arranged in series with the space current path of said amplifier tube.

8. In an electronic tube signal amplifier, the combination with a signal amplifying stage thereof having a cathode return circuit, of a gridcontrolled gas tube having input electrodes and output electrodes connected serially with said cathode return circuit, means for energizing the output electrode of the gas tube with alternating current voltage, a frequency selective input circuit for the input electrodes of the gas tube, said input circuit comprising a pair of electron discharge tubes having their space current paths arranged in series, an impedance bridgev network having a predetermined frequency rejection characteristic arranged in the output circuit of said one of said pair of tubes, said impedance network being included between the input electrodes of said gas tube, a degenerative feedback circuit connected from said impedance network to the control grid of the second of said series connected tubes, an input signal source for said amplifying stage including signals to be amplified and a controlling signal, means applying said signals to be amplified to said amplifying stage, means applying said controlling signal voltage with said signals to be amplied to the input electrode of said one of said pair of tubes, and said impedance bridge network effecting the application only of saidcontrolling signal voltage between the input electrodes of said gas tube and consequent operation of said amplifying stage for amplifying said signals to be amplified.

9. In an electronic tube signal amplifier, the combination with a signal amplier stage therein having a cathode return circuit and a signal input grid circuit, of a grid controlled gaseous discharge tube circuit providing connections [for the space current path thereof serially in said cathode return circuit, a signal supply circuit connected with said grid circuit, a low pass lter network in said connection effecting the application of signals in said supply circuit to be amplified to said inputl grid circuit, and a selective tone-responsive amplifying network coupling said gaseous discharge tube circuit with said signal supply circuit effective to apply to said gaseous discharge tube circuit a predetermined control fre-l quency included in said signal supply circuit for ring said tube and energizing said amplifier stage for the transmission of signals therethrough in response to a predetermined tone frequency.

10. In an electronic tube signal amplifier, the combination as defined in claim 9, wherein the coupling means between the gaseous discharge tube circuit and the signal input circuit includes 10 'E a shunt frequency-responsive bridge-network and an electronic tube amplifying circuit having a feedback connection through said bridge network.

GEORGE D- HANCHETI, JR.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED sTATEs PATENTS Number Name Date 1,396,745 Haddock Nov. 15, 1921 1,804,526 Coxhead May 12, 1931 2,028,859 Barton Jan. 28, 1936 2,038,683 Schramm Apr. 28, 1936 2,135,557 Beers Nov. 8, 1938 .2,139,489 Cockrell Dec. 6, 1938 2,151,740 Burrill Mar. 28, 1939 2,265,868 Schonland Dec. 9, 1941 2,368,778 Purington Feb. 6, 1945 2,382,097 Purington Aug. 14, 1945 2,392,672 Koch Jan. 8, 1946 

